Multiple output DC power supplies often are used to provide voltage to microchips and other circuit board components. Such components are sensitive to small variations in voltage; therefore, it is important that power supply voltage be carefully monitored and regulated.
In a single-output power supply, voltage may be monitored directly across the load. However, in a multi-output power supply, the negative rails of each output are connected to a common ground. The use of a common ground, while desirable to prevent damage to components, can allow different currents running to the common ground to generate strong electromagnetic interference (EMI) noise.
To alleviate the induced EMI, an operational amplifier is often used for differential sensing of the second output. The op amp blocks the loop current path, preventing EMI. Although this method is effective, as devices have been miniaturized, less space has been allocated to all parts of the device, including the power supply. Using an op amp requires at least 0.2 square inches of space. As the space allocated to the power supply is being reduced to 3.3 square inches or less, use of an op amp is becoming impractical.
What is needed is a scheme that provides for precise voltage monitoring which uses less space than an op amp, thereby permitting greater static efficiency.